Many vehicle manufacturers make a feature of the large cargo capacity of the vehicles they make.
Often this is of the "Small car, big space" theme.
However, the quoted "aircraft-hangar-like" properties of their products fall a bit short in practice, because - unsurprisingly - they're predicated
on a vehicle with nothing else in it.
Now, just about all vehicles carry a spare wheel, and a jack and wheelnut wrench (though Smart cars are a notable exception). Some also carry a warning triangle, first aid, and a fire extinguisher, as standard equipment.
We propose that when the volumetric figures are quoted, a second value is also provided based on the typical minimum equipment that needs to be there. This would include, but not be limited to, 5 litres of fuel, a litre of oil, a tyre inflator, a tow rope, a set of jump leads, a flashlight, a high-vis tabard, spare lamps and fuses, and a set of tools including a multimeter.
An additional figure could be quoted to include a pack containing a sleeping bag, a folding shovel, and some emergency food and water.
The prospective purchaser would then get a better idea of how much "space" they are actually buying ...